Prime Minister Tony Abbott has challenged key crossbenchers to provide alternative savings that "stack up", as he opens up the possibility of "adjustments" in budget negotiations.

Three months since handing down its first budget, the Government has failed to win enough support for measures such as the $7 GP co-payment, its dramatic restructuring of the tertiary sector and family benefit cuts.

On the weekend Mr Abbott said he was confident that with "an adjustment here, perhaps with an adjustment there, the vast majority of our budget measures will get through".

But the Government has not outlined what those changes might be and the Prime Minister says he will stick to the budget's "fundamentals".

"You have to deal with the Parliament that you've got and that's what the Government is doing," Mr Abbott told Macquarie Radio this morning.

"We're prepared to talk to the crossbench senators. What we're not prepared to do though is sell out the fundamentals, and the fundamentals are that we have absolutely got to get this budget crisis back under control.

"What I say to all of the crossbench senators is if you don't like what the Government is putting up, give us your alternative in terms of how we save money.

"And there were some alternatives that came up from one of the crossbench members of the Parliament, and frankly they didn't stack up for five minutes."

One key crossbench senator, Nick Xenophon, says he wants major changes to the budget.

"When the Government talks about an adjustment here, an adjustment there, it reminds me of going to the chiropractor, whereas in fact I think this budget needs some general anaesthetic and some radical surgery," he told AM.

Treasury officials have met today with the Palmer United Party, which holds three Senate seats and has formed a loose voting bloc with Victorian Motoring Enthusiast senator Ricky Muir.

But PUP leader Clive Palmer says his party's opposition to billions of dollars of budget measures has not changed.

"The budget as we see it is unfair to all Australians. It doesn't provide the opportunities for growth in the economy," he said.

"At this stage we've had no agreement between ourselves and the Government as to what we will support going into the Senate in the next couple of weeks."

Greens leader Christine Milne does not sound tempted by the call to negotiate - saying the budget is fundamentally unfair.

"No tinkering around the edges is going to fix that," she said.

"[Tony Abbott] needs to go back and actually ask the question - who can afford to contribute more in terms of revenue? And not attack the sick and the poor and the young and the unemployed."